Error using "Tone" AI ckeditor plugin [#3557012]

Problem/Motivation

I've been trying to set this up with the CKEditor Tone plugin. After successfully setting up a Chat with AI 1.2.2, when I try using the Tone option in the ckeditor I get this error in the API response:

The website encountered an unexpected error. Try again later.

TypeError: Drupal\ai_provider_google_vertex\Plugin\AiProvider\VertexProvider::loadCredentials(): Return value must be of type array, null returned in Drupal\ai_provider_google_vertex\Plugin\AiProvider\VertexProvider->loadCredentials() (line 466 of modules/contrib/ai_provider_google_vertex/src/Plugin/AiProvider/VertexProvider.php).Drupal\ai_provider_google_vertex\Plugin\AiProvider\VertexProvider->getAccessToken() (Line: 184)
Drupal\ai_provider_google_vertex\Plugin\AiProvider\VertexProvider->getAuthHeaders() (Line: 241)
Drupal\ai_provider_google_vertex\Plugin\AiProvider\VertexProvider->chat(Object, 'gemini_vertext', Array)
ReflectionMethod->invokeArgs(Object, Array) (Line: 269)
Drupal\ai\Plugin\ProviderProxy->wrapperCall(Object, Array) (Line: 133)
Drupal\ai\Plugin\ProviderProxy->__call('chat', Array) (Line: 149)
Drupal\ai_ckeditor\Controller\AiRequest->doRequest(Object, Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 741)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Set up chat with a custom Gemini.
Enable AI CKEditor and enable the Tone plugin using a vocabulary that contains a single term.
In a content form, select text, open the AI tone plugin, select a term in the vocabulary, click the "Change the tone" button.